At shallow depths, pore spaces in rocks and soil are filled with air or a combination of air and water. The water table is the level at which groundwater saturates the pore spaces of coarse rock and soils, extending to a depth where the rock porosity vanishes or the rock is molten. In coarse-grained rocks, voids in the unsaturated zone are at atmospheric pressure down to the water table, and meteoric water that percolates downward through the unsaturated zone recharges the groundwater by flowing across the water table. In such mountainous areas as the Hawaiian Islands, most of the groundwater flow collects in upgradient areas of great precipitation, so the water table of the basal aquifer near the coast is nearly a flow line, inclined at a very small angle, about 0.05 degrees to the horizon. In areas central to most Hawaiian volcanoes, groundwater flow is impeded by near-vertical basalt dikes that compartment the aquifer and cause high-level water table conditions. In some localized situations, “perched” groundwater is underlain by a layer of low-permeability rock, below which the soil or rock is unsaturated with water.
FIG. 1 illustrates a common water table condition elsewhere in the world, of modest semi-homogeneous hydraulic conductivity. In such venues, well 100 is typically drilled vertically from a position on the ground that is somewhat higher than streambed 102. Water table 104, marking the boundary between unsaturated zone 106 and water-saturated zone 108, rises gradually underneath the valley walls as the distance from streambed 102 increases. Pump 110 is positioned within well 100 at a depth that is initially below water table 104. As water is pumped to the surface through well 100, drawdown occurs and a portion of the subsurface rock and soils adjacent to well 100 becomes unsaturated with water. A local lowering or downward “coning” of the water table occurs, as indicated at 112.
Typically, the economical and practical method of groundwater production from water table aquifers is by pumping the water from vertical or nearly vertical wells drilled to depths that penetrate below the water table, as shown in the sketch section of FIG. 1. Lowering the water level in a well causes the groundwater to flow through the pores in the rock and into the well to replenish it continuously.
To produce hydrocarbons, the petroleum industry has in recent times increasingly used wells that are deviated or curved from vertical at the surface to sub-horizontal at depth, rather than wells that are vertical throughout their length. Usually, these wells originate at the surface as vertical or near-vertical wells, and at some point below the surface, the drilling trajectory curves to a shallower angle or even a horizontal attitude trajectory. Although a deviated horizontal well is more expensive to drill than a straight vertical well in some circumstances, the potential hydrocarbon recovery from a horizontal or sub-horizontal well is significantly greater than from a vertical well, and fewer wells need to be drilled. In recent years, methods and equipment have been developed to control the drilling trajectory so the deviated or sub-horizontal portion of the well traverses the desired section of rock. In particular, steerable drill bits have been developed, with the ability to control the angle from vertical as well as the compass direction in which the drilling progresses. For some applications, particularly offshore petroleum drilling, a single vertical shaft is drilled from the surface, and multiple deviated well bores are drilled outwards from the vertical shaft, using a technique called “whipstocking.” Such radial arrangements of deviated bores originating from a single vertical section have been used to penetrate specific subsurface targets, such as oil-bearing sands and remote or offshore geologic structures.
FIG. 2 illustrates two circumstances in which deviated drilling has been used for hydrocarbon production. A vertical well section 200 is drilled downwards from the ground surface 202. Deviated well bore 204 branches off of vertical section 200 to intersect an oil-bearing zone 206 near the top of a nearby subsurface geological structure 208. A second deviated well bore 210 branches off of vertical section 200 in a different radial direction to traverse a substantially horizontal sandstone reservoir 212 confined between impermeable shale beds 214 and 216.
Recently, Tesco Corporation of Calgary, Alberta, Canada, has developed a new method of deviated drilling referred to as casing drilling. In the casing drilling method, the bit is attached to the end of the casing, which is rotated from the surface. The bit can be detached and retrieved via a wireline for replacement while leaving the casing in place to support the walls of the well bore. Casing drilling facilitates penetration and retention of an intact bore through subsurface formations that are otherwise difficult to support. To date, casing drilling has economically produced more than two million feet of hole for oil and gas exploration and production.
Groundwater is generally produced from much shallower depths than hydrocarbons. The shallow water wells are generally much less expensive to drill than hydrocarbon wells, and groundwater production typically uses vertical wells. Such is the case in Hawaii. However, in situations where a fresh water lens overlies salt water within the coastal Hawaiian basal aquifer, vertical water wells may penetrate too deeply into the freshwater lens, eventually leading to upward coning and production of the underlying salt waters. When produced salt concentrations exceed drinking water or irrigation water standards, the well and, perhaps, the aquifer have to be abandoned.
It has long been recognized that a horizontal well or tunnel emplaced a small distance below the water table can skim the fresh water from a large area of such a shallow coastal aquifer, significantly prolonging the useful lifetime of the well by delaying the time when salt water contamination would end further production. However, because deviated drilling is considerably more costly than vertical well drilling, until now the method has been used rarely for water wells and nowhere for water wells in Hawaii. Deviated drilling techniques developed for petroleum exploitation have been applied for water production in some settings, such as in sedimentary rocks of the Persian Gulf, the Ogallala aquifer underlying a large part of the high plains of the United States, and the Austin Chalk formation in Texas. Since the early 1980s, Puna Geothermal Ventures and its predecessor in Puna, Hi., have used deviated wells for production of steam for geothermal use from formations 5,000 to 7,000 feet below sea level. These straight inclined steam wells were drilled from plugged vertical wells using wedges, and they deviate only about 20 to 30 degrees from vertical.
There are three different hydrogeological settings in Hawaii in which straight, substantially horizontal bores have been used to produce groundwater. The basal aquifer, a lens of freshwater floating at near sea-level upon saline water connected to the sea, has been tapped via wells on the islands of Maui (the “Maui wells”) and Oahu that were drilled or driven horizontally from near the bottom of vertical or steeply inclined shafts. These shafts were hand-excavated to positions below the water table, and the horizontal extensions from the shafts have been hand-dug or drilled. Some of the Maui wells have been copious producers for nearly 100 years.
At higher elevations, typically in elevated central parts of each Hawaiian volcano, compartmented aquifers are contained within systems of vertical basalt dikes which act as groundwater dams. Also at higher elevations are perched aquifers, occurring where a buried clay soil or impermeable ash bed forms an aquiclude. Percolating rain water provides the water supply for the compartmented and perched aquifers. In the case of the perched aquifers, inclined strata or ancient soil layers with low water permeability deflect some of the vertically percolating rain water on its way down to the basal aquifer, and the perched water may emerge at the surface as springs. Unsaturated ground occurs between the aquiclude and the next underlying water table, often the lowest or “basal” aquifer. In some cases, vertical wells have inadvertently pierced aquicludes below perched water bodies, causing water to leak across the aquicludes and decreasing the amount of water flowing in the perched aquifer.
In Oahu, west Maui, Molokai, and Hawaii, both the compartmented aquifers and the perched aquifers can feed springs where the water spills into incised valleys. The early ranchers and planters recognized the nature of the spring water sources, and in some of the deeply-incised valleys of Oahu and west Maui, they were able to drive sub-horizontal tunnels to intersect one or more dikes at levels below the water table, facilitating drawdown and the use of the reservoir capacity to sustain flow in irrigation ditches feeding the cane-fields. To tap perched aquifers, their strategy was to search for places where soils mantled ancient buried valley bottoms, with a trough in the soil layer channeling the perched waters towards the outcrop. FIG. 3 illustrates the approach followed by the ranchers. Topographic contours are indicated with solid lines 302, and the contours of a buried impermeable soil layer 304 are indicated with dashed lines. The ranchers would dig a tunnel 306 into the mountain near and above a spring 308 until the soil layer 304 was encountered at point A and then turned the tunnel parallel to the contour of the soil layer, following it into the thalweg of the ancient valley, where saturated ground lay deepest, to find a perched aquifer 310. These early Hawaiian horizontal tunnels or well bores were driven by hand mining or rotary drilling directly from nearby, steeply sloping canyon walls or from large-diameter vertical shafts. Some of these older horizontal bores are also copious producers.
In recent decades, the high cost of mining has precluded additional tunnel construction for water production from either the basal, compartmented, or perched aquifers. A scarcity of practical drilling sites and difficulties of access due to the steep terrain have left only the basal aquifers as good candidates for increasing water supplies. Meanwhile, rotary drilling technology has flourished, so essentially all new water sources have been developed by drilling vertical wells to the basal aquifer. Even in steep terrain, vertical wells have been more practical to drill than horizontal wells, because gravity aids in the vertical drilling process, while horizontal wells require special drilling technology and equipment and are, therefore, more expensive. Further, the rock formations in Hawaii are notoriously difficult to drill using rotary drill bits, such as are generally used to drill horizontal wells. No new horizontal bores of this type have been excavated in nearly 100 years, due to both the high cost of hand tunneling and to a lack of favorable sites. Thus, there is a need for a method of drilling into compartmented and perched aquifers.
Some Hawaiian basal aquifers are currently in danger of eventual abandonment due to gradually increasing salinities of waters produced through the vertical wells. For many of the municipal and irrigation wells that require large discharges, the depth of penetration into the fresh water lens has been excessive. Heavy withdrawals and drawdown have caused brackish water to enter the bottom portion of such wells, as the saline water below the lens up-cones towards the well. It is uneconomic to replace the wells of excessive depth by more numerous, shallow wells. Vertical wells provide only temporary sources of freshwater supply where drilled to the basal aquifers.
Further, when sea level rises as an inevitable consequence of global warming, the basal aquifer will rise with it. Vertical wells penetrating deeply into the fresh water lens will become contaminated sooner as the underlying salt water rises. On Maui in 2000, the water table stood 6 to 12 feet above sea level at the eleven wells producing from the Tao aquifer, so the fresh water lens may be 240 to 480 feet thick. But the average elevation of the bottom of those wells is 206 feet below sea level. Many wells tap lava tubes or bottom fairly close to the transition zone, a fact manifested by gradually increasing salinity, especially when they are produced heavily. In the worst case scenario, sea level may rise as much as 40 feet in this century, so it is likely that many more wells will have to be abandoned because their salinity exceeds potable water criteria. At the same time, groundwater supplies may be increased by heavier, more cyclonic precipitation on the islands.
Thus, there is a need for a method for producing water in volcanic terrain that is less prone to coning of salt water. There also is a need for a method for producing water from rock formations that are difficult to drill using conventional rotary drill bits. There is an additional need for a method for drilling wells originating from a vertical section of well to provide easier access to aquifers that occur as lenses above salt water or that are compartmented or perched. There is yet another need for a method of drilling wells into aquifers in a manner that maximizes water production and/or prolongs the useful lifetime of the well and the aquifer.